1. Field of the Invention
This invention relates to digital loop carrier systems and more particularly to various circuits for use therein.
2. Description of the Prior Art
Digital loop carrier systems which provide service between a multiplicity of subscribers and a telephone central office are known.
Typically, one or more subscribers are served by an electronic circuit, sometimes known as a channel unit, which provides an interface between those subscribers and the carrier system. Identical channel units are also located at the central office end of the system. While the number of subscribers served by each channel unit depends on the system architecture, a typical system servicing N subscribers has M channel units, where M is less than or equal to N.
Among the information sent by each subscriber to the central office and from the central office to the subscriber is that related to signaling. The central office must know whether each subscriber's telephone is on-hook or off-hook. The central office must send to the subscriber end of the system an indication relating to the ringing of each individual subscriber's telephone when a call has been placed to the subscriber through the system.
In earlier digital loop carrier systems, the signaling information (on/off hook, ringing) was defined at the channel units. Signaling which conveys information relating to on/off hook and ringing status requires the use of only two bits, i.e., four states. This type of signaling is known in the telephone industry as A, B signaling. In one prior art digital loop carrier system, the signaling information is sent to all of the channel units over a single line. Two other leads which are also connected to all of the channel units identify the signaling information as either A or B signaling. In addition, each of the channel units are physically wired to a channel decode block, i.e. there are address lines which are physically wired to each channel unit.
Digital loop carrier systems have become increasingly complex. This complexity has allowed larger number of subscribers to be served by the same system. Additional signaling state formats have been defined for those new systems. Further changes in transmission format standards have caused even more signaling state formats to be defined. Continuing to define the signaling information at the channel units requires a specially designed unit for each such defined signaling state format. In the context of the system described above, this also means that the channel unit card have thereon the circuitry to decode and utilize the particular signaling state format being used in the system. Alternatively, in that prior art system, it might be possible to design a single channel unit to be utilized with all known signaling state formats but such a channel unit would be quite complex.
It is, however, desirable to design a carrier system which is capable of use with any one of a number of different transmission format standards. Rather than specifically design a channel unit for each such standard or alternatively try to design a channel unit which can interface with all known standards, it is also desirable to have a system which is flexible in that it can be used with any one of a number of different defined signaling state formats. The carrier system of the present invention allows for that flexibility in that there is included therein an adaptable signaling processing unit (SPU).
The increasing complexity of digital loop carrier systems has also allowed many different types of subscribers to be served by the same system. This means that such systems have a large number of different types of channel units which can be inserted in the system in order to serve the subscriber. The particular type of channel unit inserted depends on the service needed by the subscriber. In order to keep the number of different types of channel units within reason, the channel units have the capability of being adjustable depending on the subscriber's needs. That adjustability may for example relate to the amount of gain to be provided to the subscriber.
In the prior art digital loop carrier system described above, the setting of those adjustable parameters can only be done in the field at the time the channel unit is plugged in to the slot associated with that subscriber. Such setting of the adjustable parameters is known as "provisioning". In the known carrier system, that provisioning is provided by making certain changes on the channel unit such as the setting of various dip switches on the card or moving jumpers. For that system, an installer is given an installation order indicating the particular subscriber for which the card is to be inserted, the type of card and the provisioning therefor. The installer then drives to the remote location which services that subscriber. The installer then sets the dip switches and inserts the card. The card must then be tested from the central office and in order for that to be done, the installer must place a telephone call to the office. As can be seen, the providing of service to new subscribers in the prior art digital loop carrier system is very labor intensive.
It is, however, desirable to design a digital carrier system which has the ability to both provision and test the channel units from the central office. It is also desirable to provide that provisioning and testing information to and from the particular channel unit over the same path which is used to provide the signaling to the channel unit. In order to provide all of that information to and from the channel unit, it is necessary for the unit to receive and transmit several bytes. As described above, the prior art digital loop carrier system has separate lines for the A and B signaling. If that system were expanded in order to be able to transmit and receive from the channel unit the multiple number of bits associated with the several bytes of information needed, then that system would include a large number of lines between each channel unit and the common circuitry at the subscriber end of the system. That is most undesirable. The digital transmission system of the present invention allows for that multiple amount of information to be received by and transmitted from each channel unit over only two signal paths. In the system of the present invention those two paths are shared by a multiplicity of channels.
It is also desirable to design a digital carrier system which includes therein the flexibility for interchanging time slots not only for the signaling information but also for the encoded signals to and from the subscribers of the system. Each subscriber is served by an associated channel of the system. There are several channel sequences in use today. The prior art digital loop carrier system is designed to work with only one of those sequences. In order for that system to "talk" to another of the known sequences it is necessary for that system to be provided with a map which relates the sequence for which it is designed to the other sequence. That system does not have the capability to change the channel sequence by interchanging time slots.
The digital transmission system of the present invention allows such interchanging of time slots to occur for not only the signaling information but also for the encoded signals to and from the subscribers of the system. This ability to interchange time slots allows the system of the present invention to change its channel sequence so that it can be used with any one of a number of different defined sequences. This ability to interchange time slots also allows the system of the present invention to provide certain additional features, such as the ability to test one end of the system disconnected from the other end of the system, which the prior art digital loop carrier system is unable to provide.